1. Field of the Invention
The present invention relates to an apparatus and method of controlling a three-level photovoltaic inverter, and more particularly, to a three-level photovoltaic inverter configured to separately control two DC-link voltages and an apparatus and method of controlling the three-level photovoltaic inverter.
2. Description of the Related Art
As energy problems are getting serious due to exhaustion of fossil fuels, the development of power generation systems for new and renewable energy has been accelerated, and among them, a photovoltaic generation system has been highlighted.
The photovoltaic generation system includes a plurality of photovoltaic modules, an inverter, and a controller.
Direct current (DC) power generated in the plurality of photovoltaic modules are converted into alternating current (AC) power by the inverter according to control of the controller and are supplied to a load. The reason why the plurality of photovoltaic modules are used, is that output voltages from one solar cell are very small.
That is, a photovoltaic module, i.e., a PV module is configured by generating one module by connecting several tens of solar cells or several hundreds of solar cells in series as occasion demands and by assembling the module on one panel. Since necessary voltage and power cannot be obtained using one PV module, several PV modules are connected in series or parallel.
Power that can be output by the PV module is non-linear with respect to voltage, as illustrated in FIG. 1. Thus, it is important to supply maximum power from the PV module by control. In this way, control for tracking a maximum power point of the PV module is referred to as maximum power point tracking (MPPT) control.
Output power of the PV module varies according to the intensity of solar light. Shades may occur in part of a plurality of PV modules included in one photovoltaic generation system according to weather conditions. For example, shades of cloud or other obstacles may be generated only in part of the plurality of PV modules.
Likewise, shades that occur only in part of the plurality of PV modules are referred to as ‘partial shades’. When partial shades occur, voltage-power characteristics of the PV module are changed.
FIG. 2 is a voltage-power graph of a PV module in which partial shades occur. Comparing FIG. 2 with FIG. 1, one peak is formed in FIG. 1, whereas, several peaks in FIG. 2.
In general, such partial shades occur frequently. Thus, a scheme for controlling maximum power to be supplied in case of partial shades is required. Conventional suggested techniques do not still show satisfactory results for MPPT.